1. Field of the Invention
This invention pertains to steam generators and more specifically to downhole steam generators for generating high pressure steam at the bottom of oil well bores.
2. Description of the Prior Art
The use of steam for recovering crude oil was initiated in the United States in 1960. It found its first use in the stimulation of wells drilled into reservoirs containing low gravity crude oils. Its use throughout California increased rapidly until, by the mid-sixties, the production of oil by steam stimulation exceeded 100,000 barrels a day.
Steam stimulation involves the injection of steam into a producing well for a relatively short period of time, a few days to a month or so, allowing the well to "soak" for several days or a week or two, and then returning the well to production. The steam generator is then used for injection into a second well and, in turn, a third or fourth, etc. Typically, wells are stimulated once every three months to once every year. To facilitate such operation, the steam generator was usually skid-mounted, or the steam was piped to several nearby wells that it would supply in turn.
Steam stimulation, because of the rapid production following upon the expenditure for generating steam, is an intrinsically profitable operation. The amount of oil that can be recovered from a reservoir is limited by the fact that the reach of such a technique into the reservoir is limited. As the oil is heated and drained from the zone immediately around the well bore, there is a subsequent influx of oil from the reservoir into the zone around the well bore.
The steam drive has been developed as an additional or supplementary operation to the steam soak to achieve a greater overall recovery efficiency of crude oil from the reservoir. In the steam drive, steam is injected into alternate wells (drilled in a repeating pattern) and the oil is displaced by the injected steam into the offsetting wells. Field operations have confirmed the earlier physical model studies that recovery can exceed 50% of the original oil in place, but at lower oil/steam ratios than those achieved in steam/soak operations. The lower oil/steam ratios arise from the fact that a signficantly greater fraction of the injected heat is lost because of the larger time of contact and contact area between the swept reservoir zone and the adjacent base and cap rocks.
Production of crude oil by steam stimulation and steam drive had reached some 200,000 barrels a day by 1978. The enhanced oil recovery processes are the only ones, over and above water flooding, that have proved to be economically successful to date.
The use of steam injection has been limited to date to heavy oil reservoirs that contain a very high saturation of oil, not having been depleted significantly by primary operations and water flooding. The latter, of course, is not applicable in these heavy oil reservoirs because of very adverse mobility ratio. The high oil staturation has been required so that the recovery of crude oil is sufficient to secure a significant sales volume after provision of the fuel requirements for steam generation.
Recently, attention has been placed on the extension of the steam drive to reservoirs that have been previously considered poor candidates for the process. The limits on the applicability of the steam drive arise essentially from a combination of circumstances that lead to low oil/steam ratios (oil produced/steam injected): too low an oil saturation (insufficient energy is recovered from the reservoir to provide a profitable sales volume after deducting fuel requirements), too thin a reservoir (proportionately greater fractional losses of heat to base rock and cap rock), and too deep and too high a reservoir pressure (high heat losses in the well tubulars and low steam quality at the sand face) are the principal factors limiting the extension of this scheme to crude oil reservoirs not currently amenable to the process.
This invention is aimed at removing the restraint imposed by depth and reservoir pressure on the efficiency of the steam drive operation.
In current steam drive operations, an average reservoir depth might be considered to be about 1000 feet (ranging from 500 to 2000 feet) and average injection pressures somewhere between 300 and 400 psi (ranging from 50 psi to 500 psi). Injection rates range from 500 to 2000 barrels of water (converted to steam) per day, and the steam leaves the generators at a quality of 70% to 80%. Heat losses between the generator and the sand face may run about 10% (after equilibrium conditions become established in the bore hole), and the result is that the quality of the steam is reduced to some 60% at the sand face. Higher pressures are required in order to inject the steam into higher pressure reservoirs. However, due to the fact that heat losses in the greater length of well tubulars are still greater than normal, and because the latent heat per pound of steam decreases as the sensible heat per pound increases with pressure, the quality of the steam at the sand face may fall to 40% or less.
Theoretical studies indicate that the displacement efficiency of steam decreases as the steam quality entering the reservoir decreases. This conclusion can be reached intuitively once it is realized that the residual oil saturation in a steam-filled porous medium is quickly reduced to values less than 10% of the pore volume, whereas the residual saturations to hot water are far higher (25% to 50%) and are approached only gradually. Field studies have corroborated the superiority of steam drives over hot water drives. Thus, a technically successful downhole steam generator would provide the advantages of lower heat losses in surface and downhole tubulars and a higher steam quality at the sand face. Capital and operating costs could offset these benefits and, therefore, it is the goal of this invention to provide the design of a suitable downhole steam generator that will have a positive economic ratio, i.e., benefits greater than costs.